[TEST] add version skip to weighted_avg tests

Added in 6.4, so we don't want this running on pre-6.4 BWC tests

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/260_weighted_avg.yml

@@ -1,4 +1,7 @@
 setup:
+  - skip:
+      version: " - 6.3.99"
+      reason: weighted_avg is only available as of 6.4.0
   - do:
       indices.create:
           index: test_1